For several years, nitroxides have been a major focus of research in RBB, NCI, because of their importance to radiation biology in general and their potential utility for new photodynamic therapy techniques. Electron spin resonance (ESR) is a powerful tool for free radical studies, but in the past it has been of limited value for biological work (especially in vivo) because of the excessive attenuation by water of the radio frequencies ordinarily used, about 9 GHz. The system now being developed will operate at about one-thirtieth of that frequency to alleviate the attenuation problem, and will incorporate pulse techniques developed in NMR, as well as other new techniques, to compensate for the inevitable thirtyfold loss of sensitivity.